An information recording medium for recording and reading information using a high energy density beam such as a laser beam has been developed in recent years and is now being put into practical use. The information recording medium is called an "optical disc" and is employable as video disc, audio disc or disc memory for large-capacity static image files and large-capacity computers.
The optical disc basically comprises a disc-shaped transparent substrate of a plastic material or a glass material and a recording layer provided on the substrate. An undercoating layer or intermediate layer made of a polymer material can be provided on the surface (on which the recording layer is provided) of the substrate for improving smoothness of the surface, adhesion between the substrate and the recording layer and the light sensitivity of the optical disc.
Examples of the optical discs include a disc of the ROM (Read Only Memory) type, a disc of the DRAW (Direct Read After Write) type and a disc of the erasable type.
The discs of the DRAW type and erasable type are generally provided with a groove serving as a tracking guide below the recording layer. A substrate which has not been provided with a recording layer yet and provided with only a groove is called "replica disc". On the other hand, the disc of the ROM type are previously provided with pits for recorded information on the surface of the substrate. The replica disc provided with a groove or the substrate of an optical disc provided with pits can be produced by various molding processes using an injection mold equipped with a stamper such as injection molding, compression molding, and photopolymerization methods. A recording layer is formed on the surface of the replica disc where a groove has been previously formed to obtain an optical disc of the DRAW type or erasable type. Recording of information on the optical disc can be conducted by irradiating the groove or the area between adjoining grooves with a laser beam from the substrate side to form pits or phase-altered portions on the recording layer so as to change optical characteristics of the recording layer. Reproduction of the recorded information from the optical disc can be conducted by irradiating the groove with a laser beam to detect the pits based on variation of the optical characteristics such as reflectance.
The substrates of the above-mentioned optical discs of the ROM type, DRAW type and erasable type are generally produced by an injection molding process. Accordingly, various studies have been made on the processes for producing substrates of such optical discs by the injection molding process.
The injection molding process comprises the step of injecting a molten molding resin into a mold (that is, into a cavity between a set of mold units), cooling the molten resin to form a substrate, and opening the mold to release the formed substrate from the mold.
In more detal. the generally emplyed injection molding process for producing a substrate of an optical disc comprises the steps of fixing a stamper having protrusions corresponding to pits for recorded information or a groove for tracking onto one mold unit of a pair of mold units of an injection molding machine; combining said mold units to form a closed cavity between the pair of mold units; charging a molten molding resin into the cavity; cooling the resin in the cavity to form a molded substrate; separating the mold unit having the stamper from another mold; and releasing the molded substrate from the stamper of the mold unit.
Optical characteristics of an optical disc prepared in an injection molding process for producing a substrate of an optical disc greatly depend on the conditions of the step for releasing the formed substrate. As a releasing step for obtaining a substrate of an optical disc having excellent optical characteristics, generally employed is a releasing step comprises opening the mold (i.e., separating one mold unit having a stamper on its surface from another mold unit) and subsequently supplying pressurized air between the mold surface and the resin substrate formed thereon to release the molded substrate from the stamper.
In the above-mentioned injection molding process for producing a substrate of an optical disc, the copying of the extremely small sized pattern in the form of protrusions corresponding to a pregroove and/or pits on the stamper requires high technique and high skill. Moreover, even if such copying is well made in the injection mold, the molded product should be released from the stamper uniformly in the releasing step for obtaining a substrate of an optical disc having excellent optical characteristics. In the case that the molded substrate is released from the stamper nonuniformly, the obtained substrate of an optical disc easily suffers from releasing nonuniformity (i.e., distortion of shape caused in the course of releasing), cooling nonuniformity (i.e., distortion of shape caused by cooling performed in the releasing step), etc., and hence noticeable non-uniformity of birefringence is produced, resulting in poor optical characteristics of the substrate.
Japanese Patent Provisional Publication 61(1987) -162424 discloses an improvement of the above-mentioned releasing method. In more detail, the publication proposes that a stamper coated with a precious metal such as Ag or Au on its surface is used for improving the release characteristics of the molded substrate from the stamper. However, this method has a disadvantage in that the preparation of the metal-coated stamper is complicated, and further it si difficult to uniformly release the molded substrate from the stamper all over the surface although the release characteristics can be slightly improved.
A reason why the release of the molded product cannot be made uniformly in the conventional releasing step using pressurized air or using a stamper coated with precious metal is to be as follows. When the mold is opened (that is, the mold units are separated from each other), the molded resin substrate is kept on the stamper attached to one mold unit. At that time, the molded substrate and the stamper still have a temperature higher than the surrounding atmospheric temperature. Accordingly, the molded substrate is subjected to different cooling conditions on both sides. In more detail, the molded substrate is slowly cooled on the surface in contact with the stamper, while the substrate is relatively rapidly cooled on the exposed surface (i.e., one surface in contact with the atmospheric air. Therefore, the molded substrate is apt to warp on its plane. In order to avoid occurrence of such warp of the molded substrate, it is generally employed to set temperatures of the mold units differently from each other. However, such difference of temperatures on the mold units is apt to cause a difference of the degree of thermal expansion of the molded substrate between both surfaces. Thus, the known adjustment of the thermal conditions of the mold units is still insufficient to avoid the occurrence of the various nonuniformity of the molded substrate.